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Abstract:

Disclosed is a photovoltaic device comprising: a substrate; a III-V solar
cell structure having one p-n junction on the substrate; a first
semiconductor window layer on the III-V solar cell structure; a second
semiconductor window layer on the first semiconductor window layer; an
anti-reflective layer on the second semiconductor window layer; a contact
layer disposed in the anti-reflective layer and on the second
semiconductor window layer; and an electrode on the contact layer,
wherein the second semiconductor window layer does not contain aluminum.

Claims:

1. A photovoltaic device, comprising: a substrate; a III-V solar cell
structure having one p-n junction on the substrate; a first semiconductor
window layer on the III-V solar cell structure; a second semiconductor
window layer on the first semiconductor window layer; an anti-reflective
layer on the second semiconductor window layer; a contact layer disposed
in the anti-reflective layer and on the second semiconductor window
layer; and an electrode on the contact layer, wherein the second
semiconductor window layer does not contain aluminum.

2. The photovoltaic device as claimed in claim 1, wherein the material of
the first semiconductor window layer comprises AlGaAs or AlInP.

3. The photovoltaic device as claimed in claim 1, wherein the material of
the second semiconductor window layer comprises GaP or GaInP.

4. The photovoltaic device as claimed in claim 1, wherein the thickness
of the first semiconductor window layer is in a range from 100 Å to
700 Å, and/or the thickness of the second semiconductor window layer
is less than 100 Å.

5. The photovoltaic device as claimed in claim 1, wherein the solar cell
structure comprises a single junction solar cell or a multi-junction
solar cell.

6. The photovoltaic device as claimed in claim 1, wherein the solar cell
structure comprises a dual junction solar cell which comprises a lower
battery comprising two layers of GaAs with different polarity close to
the substrate and a top battery comprising two layers of GaInP with
different polarity away from the substrate.

7. The photovoltaic device as claimed in claim 1, wherein the substrate
comprises a Ge substrate or a GaAs substrate.

8. The photovoltaic device as claimed in claim 1, wherein the
anti-reflective layer comprises a titanium dioxide (TiO2) layer
close to the second window layer and an aluminum oxide (Al2O3)
layer away from the second window layer.

9. The photovoltaic device as claimed in claim 8, wherein the titanium
dioxide (TiO2) layer comprises a thickness of from 200 Å to 800
Å, and the aluminum oxide (Al2O3) layer comprises a
thickness of from 300 Å to 1000 Å.

10. The photovoltaic device as claimed in claim 1, wherein an energy band
gap of the second window layer is not greater than that of the first
window layer.

Description:

TECHNICAL FIELD

[0001] The application relates to a solar cell device, and more particular
to a solar cell device with an improved electrical performance.

DESCRIPTION OF BACKGROUND ART

[0002] Due to the high oil price and environmental issues, the solar cells
are highly valued. Among them, the concentrated photovoltaic is most
potential to be developed. The concentrated photovoltaic mainly comprises
a solar cell composed of III-V group material. In a condition when light
is not concentrated, this type of solar cell has very high photoelectric
conversion efficiency, and shows great potential to take the place of the
conventional power sources.

[0003] Currently, an anti-reflective layer (Anti-Reflective Coating, ARC)
is generally disposed directly on the window layer in most solar cells,
as the structure shown in FIG. 1. This type of solar cell comprises a
substrate 110; a III-V solar cell structure 120 comprising one p-n
junction on the substrate 110; a window layer 130 on the solar cell
structure 120; an anti-reflective layer 150 (comprising the first
anti-reflective material layer 151 and the second anti-reflective
material layer 152) on the window layer 130; a contact layer 160 in the
anti-reflective layer 150 and on the window layer 130; and an electrode
170 on the contact layer 160.

SUMMARY OF THE DISCLOSURE

[0004] Disclosed is a photovoltaic device comprising: a substrate; a III-V
solar cell structure having one p-n junction on the substrate; a first
semiconductor window layer on the III-V solar cell structure; a second
semiconductor window layer on the first semiconductor window layer; an
anti-reflective layer on the second semiconductor window layer; a contact
layer disposed in the anti-reflective layer and on the second
semiconductor window layer; and an electrode on the contact layer,
wherein the second semiconductor window layer does not contain aluminum.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 illustrates a photovoltaic device known in the prior art
which has an anti-reflective layer disposed directly on a window layer.

[0009]FIG. 4 illustrates the testing data of the electrical
characteristics of the photovoltaic device in accordance with one
embodiment of the present application, wherein GaInP is adopted as the
material of the second window layer.

[0010] FIG. 5A illustrates the testing data of the electrical
characteristics of the photovoltaic device in accordance with one
embodiment of the present application, wherein GaP is adopted as the
material of the second window layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0011] Generally speaking, the solar cell comprising the III-V group
material has better photoelectric performance. When the electrical
characteristics of a solar cell comprising the III-V material, such as
the maximum output power density (Pmd) or conversion efficiency (η)
is assessed, the open-circuit voltage (Voc), the short-circuit current
density (Jsc), and the fill factor (FF) are particularly important. The
electrical characteristics data of the solar cell in the FIG. 1 is shown
in FIG. 2. As the data shows, this type of structure is likely to have a
great drop of the open-circuit voltage (Voc) under the high light
concentration (multi sun). According to the data, before forming the
anti-reflective layer 150, the open-circuit voltage (Voc) is 2.929V, and
after forming the anti-reflective layer 150, the open-circuit voltage
(Voc) drops to 2.875V, i.e. 0.054V lower. The open-circuit voltage (Voc)
in the high light concentration gets significantly worse. If the problem
of declining of the open-circuit voltage (Voc) in the high light
concentration is improved, the electrical characteristics of the device
can be greatly improved.

[0012] Please refer to the FIG. 3, which shows one of the embodiments of
the solar cells of the application. First, a substrate 210 is provided;
and subsequently, a III-V solar cell structure 220 comprising one p-n
junction is formed on the substrate 210; a first window layer 230 is
formed on the solar cell structure 220; a second window layer 240 is
formed on the first window layer 230; an anti-reflective layer 250 is
formed on the second window layer 240; a contact layer 260 is formed in
the anti-reflective layer 250 and on the second window layer 240; and an
electrode 270 is formed on the contact layer 260. The substrate 210 can
be a Ge substrate or GaAs substrate. The III-V solar cell structure 220
comprising one p-n junction can be a single junction solar cell or a
multi junction solar cell. In this embodiment, the solar cell structure
220 is illustrated by a dual junction solar cell structure, as shown in
the FIG. 3A, which comprises a lower battery 222 close to the substrate
210 and a top battery 224 away from the substrate 210. The lower battery
222 and the top battery 224 are connected to each other with a tunnel
junction structure 223 therebetween. The lower battery 222 comprises two
layers of GaAs layers 222a, 222b with different polarity, and the top
battery 224 comprises two layers of GaInP layers 224a, 224b with
different polarity. The tunnel junction structure 223 is composed of two
layers of AlGaAs layers 223a, 223b with different polarity. The tunnel
junction structure 223 and the lower battery 222 (or the top battery 224)
form diodes in anti-series connection electrically. In this embodiment,
the GaAs layers 222a, 222b of the lower battery 222 are p-type and
n-type, respectively. The GaInP layers 224a, 224b of the top battery 224
are p-type and n-type, respectively. And the AlGaAs layers 223a, 223b of
the tunnel junction structure 223 are n-type and p-type, respectively.
The material of the first window layer 230 may be AlGaAs or AlInP, and
its thickness ranges from 100 Å to 700 Å. The material of the
second window layer 240 is a semiconductor material which does not
contain aluminum (Al), such as GaP or GaInP, and its thickness is less
than 100 Å. The first window layer 230 and the second window layer
240 can be formed by MOCVD (Metal Organic Chemical Vapor Deposition)
method. The anti-reflective layer 250 may comprise a first
anti-reflective material layer 251 close to the second window layer 240,
which in this embodiment is a titanium dioxide (TiO2) layer with a
thickness of from 200 Å to 800 Å, and a second anti-reflective
material layer 252 away from the second window layer 240, which in this
embodiment is an aluminum oxide (Al2O3) layer with a thickness
of from 300 Å to 1000 Å. The titanium dioxide (TiO2) layer
of the first anti-reflective material layer 251 and the aluminum oxide
(Al2O3) layer of the second anti-reflective material layer 252
can be formed by evaporation using the E-gun. The material of the contact
layer 260 is a semiconductor material with a low energy band gap to
facilitate the formation of ohmic contact between the contact layer 260
and the electrode 270, and the lattice constant of the material of the
contact layer 260 is required to match that of the second window layer
240 to ensure the quality of the contact layer 260 when grown by MOCVD.
The material of the contact layer 260 can be semiconductor materials such
as GaAs and InGaAs. The material of the electrode 270 can be metal, for
example, the material selected from the group of gold, silver, aluminum,
copper, nickel, germanium, titanium, platinum, palladium, and chromium.
For a solar cell of this structure, the electrical characteristics
obtained in a testing are shown in FIG. 4 and FIG. 5, wherein FIG. 4
shows the data when GaInP is used for the second window layer 240, and
FIG. 5 shows the data when GaP is used for the second window layer 240.
As the data in FIG. 4 shows, before the deposition of the anti-reflective
layer 250, the open-circuit voltage (Voc) in the high light concentration
(multi sun) is 2.968V, and after the deposition of the anti-reflective
layer 250, the open-circuit voltage (Voc) drops to 2.959V, by only
0.009V. In contrast with the structure in the FIG. 1 which has a
declining value of 0.054V, the open-circuit voltage (Voc) of the
structure in this embodiment in the high light concentration (multi sun)
is significantly improved. And as the data in FIG. 5 shows, before the
deposition of the anti-reflective layer 250, the open-circuit voltage
(Voc) in the high light concentration (multi sun) is 2.947V, and after
the deposition of the anti-reflective layer 250, the open-circuit voltage
(Voc) drops to 2.939V, by only 0.008V. In contrast with the structure in
the FIG. 1 which has a declining value of 0.054V, the open-circuit
voltage (Voc) of the structure in this embodiment in the high light
concentration (multi sun) is significantly improved. These results show
that the structure in FIG. 3, where the second window layer 240 is formed
on the first window layer 230, prevents the open-circuit voltage (Voc) of
the device in the high light concentration from drastically dropping. In
addition, the short-circuit current density (Jsc) is kept at a
substantially same level, and the device has a better electrical
characteristics. A calculation of the maximum output power density,
wherein the maximum output power density (Pmd)=open-circuit voltage
(Voc)×the short-circuit current density (Jsc)×the Fill Factor
(FF), shows that for the structure in the FIG. 1 in the high light
concentration (multi sun), before the deposition of the anti-reflective
layer, the maximum output power density
(Pmd)=2.929×1762.083×0.892=4606.373 (mW/cm2). Similarly,
after the deposition of the anti-reflective layer, the maximum output
power density (Pmd)=5540.643 (mW/cm2). The ratio of the maximum
output power density (Pmd) after the deposition of the anti-reflective
layer to the maximum output power density (Pmd) before the deposition of
the anti-reflective layer is 1.203(=5540.643 (mW/cm2)/4606.373
(mW/cm2)). That is, the maximum output power density (Pmd) increases
20.3%. In contrast, a similar calculation is done based on the data in
FIG. 4 and FIG. 5, and the maximum output power density (Pmd) increases
33.8% and 33.6% after the deposition of the anti-reflective layer,
respectively. This result shows a great increase in comparison with the
20.3% for the structure shown in FIG. 1. The cause of this result is that
the material of the second window layer 240 is a semiconductor material
which does not contain aluminum (Al). For example, the materials such as
GaP or GaInP in the embodiment of the present application are materials
without aluminum (Al). It can prevent the aluminum (Al) element in AlGaAs
or AlInP of the first window layer 230 from reacting with the TiO2
layer of the anti-reflective layer 250 during the evaporation process of
the anti-reflective layer 250 which comprises the TiO2 layer and the
Al2O3 layer by adding the second window layer 240 without
aluminum (Al) on the first window layer 230. Otherwise the aluminum (Al)
element in AlGaAs or AlInP of the first window layer 230 would react with
the TiO2 layer to oxidizing the aluminum (Al) element and cause low
Voc. Accordingly, the damage to the first window layer 230 (AlGaAs or
AlInP) by the evaporation process of the anti-reflective layer 250 is
reduced, and the device is so protected to have a better efficiency.
Based on this principle, it is understood that it is not necessary for
the energy band gap of the second window layer 240 to be higher than the
energy band gap of the first window layer 230. That is, the selection of
the material for the second window layer 240 is not limited by the
criterion of the energy band gap. A material of the second window layer
240 with an energy band gap greater than that of the first window layer
230 may be selected, and a material of the second window layer 240 with
an energy band gap not greater than (i.e., less than or equal to) that of
the first window layer 230 may also be selected. The thickness of the
second window layer 240 should not be too thick, and is preferably lower
than 200 Å, and the most preferably lower than 100 Å, so the
incident light is not absorbed to harm the performance of the solar cell.
In addition, although a lattice mismatch still occurs between a thinner
second window layer 240 and the window layer 230 below, a thinner second
window layer 240 is easier to have an elastic deformation to make its
lattice constant consistent with that of the window layer 230 below. If
the second window layer 240 is too thick, the second window layer 240 has
a large stress and tends to revert to its original lattice constant,
which causes lattice defects. So, a thinner second window layer 240 has a
better tolerance to the lattice constant mismatch to the first window
layer 230.

[0013] The above-mentioned embodiments are only examples to illustrate the
theory of the present invention and its effect, rather than be used to
limit the present invention. Other alternatives and modifications may be
made by a person of ordinary skill in the art of the present application
without escaping the spirit and scope of the application, and are within
the scope of the present application.